Foam molded polyethylene resin bodies are widely used, for example, as a shock absorbing packing material. In particular, as a shock absorbing material used for products to which dirt or dust undesirably adhere, such as an electronic component, an antistatic foam molded polyethylene resin body is used. Such an antistatic foam molded polyethylene resin body is produced from foamed polyethylene resin particles containing an antistatic agent.
The foam molded polyethylene resin body produced from foamed polyethylene resin particles unfortunately has a large shrinkage ratio with respect to mold dimension and poor dimensional precision. These problems are likely to be markedly caused by the foamed polyethylene resin particles produced from a polyethylene resin containing an antistatic agent.
In order to solve the problems, various studies have been carried out. The studies are exemplified by a method of imparting foaming performance to foamed particles by adding various molding gases (Patent Document 1), a method of imparting foaming performance by compressing foamed particles (Patent Document 2), a method of adjusting the expansion ratio of pre-foamed particles and foamed particles used for molding within a predetermined range (Patent Document 3), and a method of giving a complicated temperature history after molding (Patent Documents 4 and 5).
However, these methods have advantages and disadvantages involving many equipment limitations and problems to be solved.
For example, the method described in Patent Document 1 by adding a molding gas requires an apparatus for adding the gas to foamed particles, and the gas addition apparatus and a molding processing are required to be integrated or to be continuously arranged in order not to reduce the foaming performance imparted.
The method described in Patent Document 2 by compressing foamed particles requires an apparatus for compression, and the compression apparatus and a molding machine are required to be integrated. In addition, the method produces defective products at a higher rate than the case without compression because foamed particles have poor packing properties.
The method described in Patent Document 3 by adjusting the expansion ratio of pre-foamed particles and foamed particles used for molding requires the preparation of the pre-foamed particles having an excessively high expansion ratio. In addition, in order to adjust the ratio, particles are required to be left in a temperature environment at about 60° C. for several hours so as to give a desired ratio.
The methods described in Patent Documents 4 and 5 by giving a complicated temperature history after molding complicate operations and thus are not an advantageous method for production processes.
In order to solve these problems in processes, a method of using polyethylene glycol or glycerin has been developed (Patent Documents 6 and 7). Patents employing an antistatic agent are also disclosed and include, for example, a method of employing a fatty acid glyceryl ester having an HLB value of 3 or more and less than 4 and a melting point of 35° C. or more and 75° C. or less (Patent Document 8) and a method of adjusting a melt index, a melt tension, and a cell size within particular ranges (Patent Document 9).
The methods in Patent Documents 6 and 7 have no problem concerning the shrinkage ratio with respect to mold dimension but have a problem of poor surface stretch. The method described in Patent Document 8 has no problem concerning staining and antistatic properties but produces a molded body having a large shrinkage ratio with respect to mold dimension. The method in Patent Document 9 has no problem concerning the shrinkage ratio in the thickness direction but has a problem of large shrinkage ratios with respect to mold dimension in the longer direction and in the shorter direction.
As described above, it is difficult to simply obtain non-crosslinked foamed polyethylene resin particles capable of affording an antistatic foam molded polyethylene resin body that has a small shrinkage ratio with respect to mold dimension, is deformed in a small degree, and has good surface stretch by a simple method.